In recent years, ion sensors and biosensors have been developed so as to measure the concentration of chemical species in liquid media to be examined. Such sensors are generally referred to as chemical sensors, and comprise a sensitive membrane to which a cyclic compound--called an ionophore--for specific binding to a chemical species to be measured, an enzyme for specifically catalyzing the reactions involved and like substances are fixed, and a transducer portion for transducing the quantity of the material specifically selected thereby into an electrical signal. Such chemical sensors have been applied to clinical, environmental, food and other inspections. As an example of one such transducer, an ion sensitive field-effect type transistor (hereinafter abbreviated as ISFET) is available.
This ISFET makes use of a change in electrical conductivity in the vicinity of a semiconductor surface, which occurs depending upon a change in the interfacial position of a sensitive membrane provided on the surface of the semiconductor and a solution.
Referring to the ISFET illustrated in FIG. 13, by way of example, a single crystal 1 of silicon is doped to form thereon a source electrode 2, a drain electrode 3 and a channel layer 4 located therebetween. An insulating layer 5 of SiO.sub.2 is then provided over them to form a field-effect type transistor (hereinafter often abbreviated as FET). Further, a water-resistant and hydrogen ion-sensitive membrane 6 of Si.sub.3 N.sub.4, etc. is deposited on the insulating film of SiO.sub.2. As a gate voltage is impressed between the thus constructed assembly while immersed in a solution and a separate reference electrode provided therein through these films, a certain voltage is applied between the source electrode and the drain electrode to induce a carrier in the channel layer, thereby providing a drain current flow. An effective gate voltage is changed by the interfacial potential between the hydrogen ion sensitive film and the pH of the solution, whereby a change in the drain current can be detected.
However, the ISFET including the FET and the ion sensitive membrane made integral therewith has been incompatible in terms of the water resistance and sensitivity of the ion sensitive membrane, since an intended increase in the sensitivity results in a decrease in the water resistance and vice versa.
For that reason, there has also been known the so-called extended gate type ISFET wherein, as illustrated in FIGS. 14 and 15, a p type silicon substrate 11 is provided thereon- with a SiO.sub.2 layer, a signal transmission line 17 defined by an InO.sub.2 film is formed at a position on the same substrate, which is different from the position of an FET portion 15 having drain, source and gate electrodes 12, 13 and 14, while it is connected to the gate electrode 14, and an ion sensitive portion, having ion sensitive membrane 16, etc. are formed on a part of the line 17. This extended gate type ISFET is used in combination with a separate reference electrode provided in a solution to be examined, while its ion-sensitive portion alone is immersed in that solution.
However, the extended gate type ISFET shown in FIGS. 14 and 15 incurs a rise in the production cost due to unsatisfactory efficiency of utilization of an expensive silicon substrate. This is because the ion-sensitive membrane portion that need not in principle be mounted on a silicon substrate and the FET portion that need be mounted on a silicon substrate are formed on the same silicon substrate.
Another problem with the ISFET of such a type is that it is unsuitable for the measurement of a slight quantity, i.e., about 0.05 to 0.1 ml of a sample under examination, as encountered in medical inspections.
A further problem with the extended gate type ISFET was that when it was used as a disposable chemical sensor by reason of the re-use of the sensitive membrane being unpreferred, the FET accounting for a substantial part of the cost of the chemical sensor was wasted, since the re-usable FET portion was discarded together with the membrane.
The chemical sensor need be of good accuracy and reproducibility. For that purpose, the properties of the FET must be uniform. To use a new FET for each measurement of a liquid under examination gives rise to a variation in the performance of the chemical sensor, since variations in the properties of the FET's produced are unavoidable. This leads to a still further problem that any data of good accuracy cannot be obtained.
One type of the extended gate type chemical sensor is disclosed in Japanese Patent Laid-Open Publication No. 63 (1988)--128254. According to this sensor, however, an FET portion of a measuring circuit is formed separately from an electrode portion to be immersed in a sample under examination, and is not of any flat structure.